Catalytic cracking produces about half of the gasoline which is sold in the United States. Although catalytic cracking was originally introduced as a cyclic fixed-bed process, fluid catalytic cracking is now the significant mode of operation. In the fluidized process, small particles of catalyst are suspended in upflowing gas to be handled like a liquid and circulated between reaction and regeneration vessels. The purpose of catalytic cracking is to convert heavy vacuum gas oil or resid to compounds of lower molecular weight boiling in the gasoline and middle distillate range. The primary object is to increase gasoline yields and to raise octane numbers. Gas oil from catalytic cracking is olefinic.
Broad process parameters include temperatures ranging from about 480 and 510.degree. C. in the reactor to about 620.degree. C. in the regenerator when synthetic silica-gel catalysts activated with 15-60% Al.sub.2 O.sub.3 is employed. Operating pressures are 150-200 kpa (22-29 psi). Zeolite catalysts withstand higher temperatures and are accordingly regenerated above 700.degree. C. to gain the yield benefit owing to lower carbon make (less than 0.1%). Increasing residual carbon content on regenerated catalyst has the effect of decreasing gasoline make, and decreasing C.sub.5.sup.- fractions, while increasing heating oil production.
The amount of coke burned in the regenerator can range from 3 to 6 wt. % based on the fresh feed. That burning results in a release of a large amount of heat. Excess heat is transferred to the reactor by means of circulating catalyst. The circulating catalyst leaving the reactor is stripped with a stripping medium, often steam, to recover hydrocarbons before regeneration occurs. Of the fresh feed, 70 to 80% is cracked to lower boiling materials. With the newer more active zeolite type catalyst, conversion results in increased cracking, up to 80 to 90%.